1. Field of the Invention
The present invention relates to a superacid catalyst based on sulfated zirconium oxide, the process for its preparation and its use in acid-catalyzed processes and in the hydroisomerization of n-paraffins.
2. Description of the Background
Catalysts based on sulfated oxides of zirconium, titanium, iron having superacid characteristics, according to the definition of Gillespie, as described for example by K. Arata, Adv. Catal., 37, 165, 1990, are known in the art. These superacid catalysts are usually prepared by means of an articulate synthesis comprising numerous steps. For example sulfated zirconia (ZrO.sub.2 /SO.sub.4.sup.-2) is generally prepared as follows:
1) precipitation of fresh zirconia hydroxide; PA1 2) drying; PA1 3) impregnation with a sulfating agent; PA1 4) calcination.
Step (3) can be carried out in different ways: via wet inbibition point, as described in EP 520 543, by treatment with a gaseous stream of H.sub.2 S or SO.sub.2, as described by J. R. Shon, H. W. Kim, in J. Mol. Catal., 52, (1989), 361, or by treatment in solution (R. Le Van Mao, S. Xiao and T. Si Le, Catal. Lett., 35 (1995), 107; D. Farcasiu, J. Qi Li, in appl. Cat. A, 128 (1995), 97). All the steps of this preparation are critical: zirconium precursor, drying temperature, sulfating agent, concentration of the solution of sulfating agent used, temperature and calcination time. It is therefore difficult to control and reproduce the synthesis. Simplified syntheses of ZrO.sub.2 /SO.sub.4.sup.2- comprising a single synthesis step, have recently been effected. For example H. Arata et al. use Zr(SO.sub.4).sub.2 as precursor (Bull Chem. Soc. Jpn., (1990), 244): this method however does not allow the content of sulfur and its dispersion to be controlled. U. Ciesla et al. precipitate the zirconium hydroxide in the presence of alkyl sulfonates or sulfonates (EUROPACAT II Congress 3-8 September 1995). The crystallization of the amorphous phase starts at very high temperatures, over 650.degree. C.
Another synthesis method in a single step is based on the gelification of Zr(OC.sub.3 H.sub.7).sub.4 dispersed in propanol, in an acid environment of HNO.sub.3 /H.sub.2 SO.sub.4. The material, before being calcined, must be dried under supercritical conditions (D. A. Ward, E. I. Ko, J. Cat., 150, (1994), 18).
In D. Tichit et al., Catal. Let., 38 (1996) 109-113, Zr(OC.sub.3 H.sub.7).sub.4 dispersed in propanol, is gelified in an acid environment of H.sub.2 SO.sub.4. The materials obtained after calcination at 650.degree. C. prove to consist of tetragonal phase associated with small quantities of monocline phase.